The present invention relates to an apparatus for inclusion in a ship to reduce vibrations in the ship's hull and to a ship incorporating such an apparatus. The present invention additionally relates to a method of reducing vibrations in a ship's hull.
It is widely accepted that hull vibrations are capable of giving rise to significant stresses within the structure of a ship which may, in time, lead to structural fatigue and ultimately structural failure. The elmination or reduction of these vibrations is therefore of considerable interest to ship designers.
Hull vibrations span a very large frequency range from the movement of the ship in the seaway through the low frequency hull girder vibrations and the various forms of engine vibration to the higher frequency local vibrations. The means used to eliminate or reduce these vibrations are similarly diverse. The movement of a ship in the seaway, for example, is generally counteracted by the use of stabilisers of which the Denny-Brown is probably the best known design offered commercially. The design relies on the use of hydrofoils on either side of the ship which provide moments that compensate for the action of the waves and thereby reduce the rolling of the ship.
Hull girder vibrations on the other hand, have until recently been thought to be caused exclusively by excitations from the main engines, propellors and other machinery contained within the ship. In particular it has been thought that these vibrations were predominantly excited by the primary and secondary out of balance of the main engine. Balanced main engines have therefore often been fitted to ships to minimise these hull girder vibrations, commonly in combination with a Nishishiba balancer which is used to balance the second order out of balance of the main engine.
More recently however, a better understanding of the importance of wave excitation has been developed and it is now recognised that lower modes of hull girder vibration may also be excited by the action of waves. In a recent experiment conducted during a ship's sea trials a wave-excited, two node mode of vibration was detected at the after end of the ship's hull having an amplitude of 1.78 mm and a frequency of 1.48 Hz, and this on a day when the weather was good and the sea relatively calm. It was calculated that the detected vibration gave rise to a nominal vibration stress in the deck of 2.5N/mm.sup.2 while further measurments suggested that the speed of the ship was adversely affected.
It is now thought that the hull girder vibrations may also be excited by processes known as "bottom slamming" and "bow flare slamming" in which, as waves break over the bow of the ship, the buoyancy of the bow section is alternately decreased and increased setting up a transverse standing wave throughout the length of the hull. The amplitude of the vibration is a maximum at the bow where the excitation occurs but can give rise to significant stresses throughout the ship, particularly at deck level both because of the presence of hatches and also because the double bottomed nature of a typical hull provides a much stronger structure. For example the two node mode of vibration in which the wavelength is equal to the length of the ship and both bow and stern are antinodes is thought to give rise to stresses at least as large as those calculated on the basis of the rigid body assumption.
The frequency of hull girder vibrations are primarily determined by the structural stiffness and mass of the hull. Because mass plays a part in determining the frequency of the vibrations it will of course mean that the hull of a ship when loaded will have a different natural frequency than when unloaded. Typically for longer ships however, the frequncy of vibration of the two node mode is between 0.6 Hz and 1.0 Hz whilst for smaller ships the frequency of vibration may be increased to nearer 2.0 Hz. Since under typical sea conditions waves are present from a broad frequency spectrum, it is to be expected that there will always be some waves present capable of stimulating this two node mode of vibration. As ship designers can have no control over the excitation of these vibrations it is considered that the best way of minimising the resulting stresses is to reduce the amplitude of the vibrations by means of damping.
At higher frequencies, modes of hull girder vibration become difficult to distinguish from local vibrations. In this frequency range the most important excitations are usually harmonics of the blade frequency, that is the product of the number of blades provided on the propeller and its rotational speed, and orders corresponding to the number of cylinders in the main engine. These vibrations are relatively difficult to predict with accuracy but as far the propeller is concerned, the vibrations may be limited by using relatively large propeller clearances. This can however result in some loss of propulsive efficiency. As for the engine induced vibrations, axial vibration dampers and various forms of engine stays are just some of the controling means available to the ship designer. The literature available in the art would suggest that vibrations in this frequency range seldom cause major problems but further damping could clearly enhance ship development.